As the Army struggles to meet recruitment numbers, FRONTLINE takes a hard look at private contractors servicing U.S. military supply lines, running U.S. military bases, and protecting U.S. diplomats and generals. Between the logistics giant Halliburton and a myriad of armed security companies, private military contractors comprise the second largest "force" in Iraq, far outnumbering all non-U.S. forces combined. There are as many as 100,000 civilian contractors and approximately 20,000 private security forces.

In "Private Warriors," FRONTLINE correspondent Martin Smith travels throughout Kuwait and Iraq to give viewers an unprecedented behind-the-scenes look at companies like Kellogg, Brown & Root, a Halliburton subsidiary, and its civilian army. KBR has 50,000 employees in Iraq and Kuwait that run U.S. military supply lines and operate U.S. military bases. KBR is also the largest contractor in Iraq, providing the Army with $11.84 billion dollars in services since 2002.

Historically, there is nothing new about the military's use of private contractors, but the Iraq war has seen outsourcing on an unprecedented scale. The policy change came after the Cold War when the Pentagon was downsizing under then Secretary of Defense Dick Cheney. Cheney first hired Halliburton as a consultant and later became the company's president. Halliburton subsidiary KBR is now one of the largest recipients of government contracts.

FRONTLINE visits the biggest Halliburton/KBR run base, Camp Anaconda, in the Sunni triangle. Behind concrete walls 28,000 soldiers and 8,000 civilians live in bases that offer Taekwondo and Salsa lessons, movie theatres, fast food courts, and four meals a day. The amenities are impressive, but some argue that there is a price to pay. Says a former base commander Marine Colonel Thomas X. Hammes, "it's misguided luxury … somebody's risking their lives to deliver that luxury."

And while KBR was glad to provide Smith with a tour of the facilities, they weren't able or willing to answer some basic questions about how much certain services -- like feeding the troops -- cost. Smith eventually finds some answers from the Army base commander, but numerous audits are underway to determine just how the contracts are being fulfilled. In response to allegations of overcharging in the tens of millions of dollars, KBR's Vice President of Worldwide Military Affairs, Paul Cerjan says, "the only thing we can do is stand up and give a true and honest evaluation of what we've done. … And let whoever is making the assessment make the assessment. We are not afraid of that process."

"Private Warriors" also explores a very different kind of contractor -- the private world of security teams that work for firms like Blackwater, Aegis, and Erinys. They provide armed protection for U.S. government officials, government offices, military installations and even military commanders.

"The Pentagon's increasing reliance on outsourcing military functions raises important questions about accountability and the chain of command," says Smith. Through conversations with top military commanders, policy planners, military experts, and contractors, "Private Warriors" explores some of the dangers in bringing in the private sector to prosecute the war.

Warns George Washington University Professor Steve Schooner, an expert on military contracting, "We have tens of thousands of armed contractors in Iraq defending the Green Zone, defending the military, defending contractors… But they're not part of the military command structure." Schooner suggests there can be trouble when private contractors carry weapons and have tactical responsibilities yet aren't getting the same information or direction. Peter Singer, a fellow at the Brookings Institution and author of "Corporate Warriors" agrees: "There's a bubbling resentment … and you're starting to sense a backlash from the military."

Smith obtains unusual access to Erinys, a British private security company. They have been charged with protecting the U.S. Army Corps of Engineers and are paid $50 million a year for the task. Erinys is staffed with an assortment of ex-Special Forces and policemen from around the world. A private security guard at Erinys makes approximately $400 dollars a day, twice what a soldier makes. Some guards make up to $1000 a day. While some see these men as hired guns, they do not view themselves that way. They say they are just men with more expertise than the military when it comes to protection. If Andy Melville, a project manager with Erinys in Iraq is correct, private warriors could become more prevalent in Iraq.

"Americans would like to withdraw troop members," says Melville. "And perhaps it is part of their policy to reduce troop members and replace them with private security contractors."

Given the recent difficulties in obtaining enough recruits for the Army, FRONTLINE asked the Pentagon for a high-ranking official to discuss if there are plans to place greater reliance on private warriors and to address other questions about accountability and costs. The Pentagon declined to provide anyone to be interviewed after acknowledging this was a sensitive issue.

Like regular soldiers, security contractors have a high profile, and increasingly, find themselves being targeted by insurgents and have suffered casualties as a result. Indeed, the dangers of this war come into sharp focus in the film. A security guard with another company, who accompanies Smith on a run to a reconstruction site north of Baghdad, was killed less than 24 hours later by a suicide car bomb. The next day, Blackwater lost six men when a helicopter carrying their employees was shot down by a rocket-propelled grenade. In a spike of violence, 18 private contractors were killed during the two-week period the FRONTLINE team was in Iraq.

"Private Warriors" also reexamines one of the most shocking episodes of the war, when the charred remains of Scott Helvenston, Wes Batalona, Jerry Zovko, and Michael Teague -- killed while protecting a truck convoy headed to pick up kitchen equipment -- were desecrated and strung over a bridge in Fallujah in March 2004. FRONTLINE follows their story and speaks to close personal friends and family who claim that the four men were poorly equipped for the mission, lacking vehicle armor and sufficient manpower to fend off an attack.

In addition to lingering questions about accountability in the incident, Marine Colonel John A. Toolan, who was ordered into the city to find the killers, found himself forced to change his original plan for quelling hostilities. The private contractors had gone in unbeknownst to the Marines. Colonel Toolan tells FRONTLINE, "Certainly the next time … I'm sent some place with Marines, the amount of control that I need to establish up front is gonna be clear."

"Private Warriors" ends on the infamous Baghdad Airport Road. In the last four months there have been at least 150 attacks on this 5-mile stretch. So far the U.S. military and their partners in the private sector have been unable to secure this vital link. Smith has his own private security guards as he travels along what Iraqis call Death Road. He leaves behind a country where the fact that business for the private contractors is booming signals how difficult and dangerous the situation remains.